Add support for automatically gitignoring generated files

Add a light `gitignore` abstraction allowing to define a set of
gitignore patterns from Nickel, and plug that in to the file generation
interface so that a generated file can also be git ignored by simply
setting `files.foo.gitignore = true`

.gitignore

@@ -1,2 +1,6 @@
+##########
+# This file is autogenerated by Organist, don't edit
+##########
+.editorconfig
 /examples/*/result
-/result
+/result

lib/git.ncl

@@ -0,0 +1,52 @@
+# let nix = import "./nix-interop/nix.ncl" in
+let filegen = import "./files.ncl" in
+let Gitignorable = {
+  gitignore
+    : Bool
+    | doc "Whether to gitignore the file"
+    | default
+    = false,
+  ..
+}
+in
+let header : String = m%"
+    ##########
+    # This file is autogenerated by Organist, don't edit
+    ##########
+
+  "%
+  in
+let ignored_files | { _ : Gitignorable } -> { _ : Bool } = fun files =>
+    files
+    |> std.record.to_array
+    |> std.array.filter (fun { value = { gitignore, .. }, .. } => gitignore)
+    |> std.array.map (fun { value = { target, .. }, .. } => { field = target, value = true })
+    |> std.record.from_array
+  in
+let gitignore_content | { _ : Bool } -> String = fun ignores =>
+    ignores
+    |> std.record.map
+      (
+        fun pattern is_ignored =>
+          "%{if is_ignored then "" else "!"}%{pattern}"
+      )
+    |> std.record.values
+    |> std.string.join "\n"
+    |> (fun c => header ++ c ++ "\n")
+  in
+{
+  Schema =
+    filegen.Schema
+    & {
+
+      files | filegen.Files,
+      files | { _ : Gitignorable },
+
+      git.ignore
+        | { _ : Bool }
+        = ignored_files files,
+
+      files.".gitignore".materialisation_method = 'Copy,
+      files.".gitignore".content = gitignore_content git.ignore,
+    }
+}

lib/schema.ncl

@@ -1,5 +1,6 @@
 let nix = import "./nix-interop/nix.ncl" in
 let filegen = import "files.ncl" in
+let git = import "git.ncl" in
 {
   OrganistShells = {
     dev | nix.derivation.NickelDerivation = build,
@@ -30,4 +31,5 @@ let filegen = import "files.ncl" in
       ..
     }
     & filegen.Schema
+    & git.Schema,
 }

project.ncl

@@ -74,12 +74,21 @@ let import_nix = organist.nix.import_nix in
 
   flake.checks = import "tests/main.ncl",
 
-  files.".gitignore".materialisation_method = 'Copy,
-  files.".gitignore".content = m%"
-    # This file is generated by Organist, please don't edit directly
-    /examples/*/result
-    /result
+  # Simple config file. It will be gitignore and regenerated on the fly by the
+  # `#regenerate-files` command.
+  files.".editorconfig".content = m%"
+    # Auto-generated by Organist, don't edit
+
+    root = true
+    [*.{nix,nickel}]
+    indent_style = spaces
+    indent_size = 2
 
   "%,
+  files.".editorconfig".gitignore = true,
+
+  # Extra gitignored files
+  git.ignore."/result" = true,
+  git.ignore."/examples/*/result" = true,
 }
   | organist.OrganistExpression